1. Field of the Invention
The invention relates to the frequency tuning circuit, more particularly to a frequency-tuning loop in the Transconductor-Capacitor filter.
2. Description of the Prior Art
A filter is a common but important unit in a general signal processing system, the function of which is to eliminate the unnecessary band and to preserve or amplify the necessary ones. In the production process of advanced integrated circuits, it is a general and common trend to integrate filters into the design of the chip.
The cut-off frequency and the reciprocal of the time constant of the filter are in direct ratio. The time constant in the active RC filter is symbolized by the R.C value and that in the transconductor-capacitor filter is symbolized by the C/Gm value. However, due to the uncertainty of the integrated-circuit-fabrication process, the resistance value, the capacitance value, and even the value of resistance value multiplying capacitance value produced, the most important in the filter, range greatly, and therefore the frequency precision of the filters produced is below standard or unstable.
Therefore, a frequency-tuning loop is needed to set the filter, which can focus on a signal-inputting resource (such as a constant clock signal), to measure the time constant, and to alter or adjust the cut-off frequency of the filter.
A Transconductor-Capacitor filter is a common filtering technique that has a high-speed feature. The most important thing in designing a Transconductor-Capacitor filter is to make sure that the cut-off frequency is controlled in a designed range that does not change with the production process, temperature, or time. There are two common controlling methods, one is to make the calibration to the cut-off frequency of the filter according to the inputting clock, which can repair all the differences in the production process. Having the advantage, that once it is calibrated it will not affect the filter anymore. The disadvantage being, that precision in the calibration is limitedxe2x80x94incapable of doing repairs to the variables such as temperature changing, time aging, etc. At the same time; the other method uses a continuous tuning to the cut-off frequency of the filter according to the consistency of the inputting clock signal, having the advantage that it can do repairs to the variables such as temperature changing, time aging, etc. But also the disadvantage is that the offset current produced by the Transconductor will affect the adjusted results and therefore lower the degree of precision needed.
As shown in FIG. 1A, a block diagram of a prior Transconductor-Capacitor filter with a frequency-tuning loop, after the inputting signal goes through the Transconductor-Capacitor filter 110, there is a filtered outputting signal, and this Transconductor-Capacitor filter 110 has a frequency-tuning Loop 112 that receives a fixed clock and generates tuning-voltage for the Transconductor-Capacitor filter 110 to adjust the outputting signal according to the calibration of this clock.
FIG. 1B is a model diagram of the interior circuit of the frequency-tuning loop 112 in FIG. 1A, a tuning circuit designed according to xe2x80x9ca novel approach for the automatic tuning of continuous-time filtersxe2x80x9d in IEEE proc. ISCAS-91, the idea of which is to use the charge-transforming negative-feedback loop. The transconductor 120 itself to form the resistance   R  =      1    Gm  
of equal effect with the negative-feedback while the voltage produced by the current supply Ir from the positive end of the Transconductor 120 going through Gm is   V  =                    I        r            Gm        .  
The on and off of the first switch 122 and the second switch 123 are respectively controlled by the clock and their on-off conditions are different. When the check controlling first switch 122 is high-level then conductive, the capacitance C1p is filled with charge Qp, and             Q      p        =                            C                      1            p                          *        V            =                                    C                          1              p                                *                      I            r                          Gm              ,
and at this time the second switch 123 is off and in open-circuit condition, which makes the capacitance C1p and the later circuit unable to affect each other. And when the high-level of the clock turns to low-level, the first switch 122 is switched off and becomes an open-circuit, and the second switch 123 conductive, and at this time the charge Qp deposited in the capacitance C1p is transformed to outputting voltage             Δ      ⁢              xe2x80x83            ⁢              V                  cp          ,                    ⁢      Δ      ⁢              xe2x80x83            ⁢              V        cp              =                            -                      Q            p                                    C                      2            ⁢            p                              =                                    -                          C                              1                ⁢                p                                                          C                          2              ⁢              p                                      *                              I            r                    Gm                      ,
on the integrator circuit constituted by integrator 124, the capacitance C2p, and the capacitance C2n.
Moreover, since there is another current supply 125 on the inputting end of the integrator 124, the current value of which is N*Ir, therefore the outputting of the integrator increases constantly, and in a unit of clock period T=1/f, the increased voltage of the integrator 124 due to the current resource 125 is       Δ    ⁢          xe2x80x83        ⁢          V      1        =                    I                  C                      2            ⁢            p                              *      T        =                            N          *                      I            r                                    C                      2            ⁢            p                              *                        1          f                .            
And when the negative-feedback loop reaches balance in the end, xcex94Vcp+xcex94V1=0, the Equation 1 can also be reached:                                                                         -                                  C                                      1                    ⁢                    p                                                                              C                                  2                  ⁢                  p                                                      *                                          I                r                            Gm                                +                                                    N                *                                  I                  r                                                            C                                  2                  ⁢                  p                                                      *                          1              f                                      =                              0            ⁢                          
                        ∴                          ⇒                              Gm                                  C                                      1                    ⁢                    p                                                                                =                      f            N                                              (Equation  1)            
Therefore, we can see that the tuning-frequency of the frequency-tuning loop 112 can be controlled with its f/n value, thus the cut-off frequency of the Transconductor-Capacitor filter can be tuned to the designed value with this frequency-tuning loop 112.
Besides, the units connected to the other inputting end of the transconductor 120, such as the third switch 126, the fourth switch 127, and the capacitance C1n, C2n, have similar way of connecting among each unit itself, the principle of action, and other units as the way described above, and only that it is connected to the other end so the outputting voltage xcex94Vcn is different from xcex94Vcp by a negative sign. Then xcex94Vcn and xcex94Vcp outputted after going through a differential to single converter 128 and a low-pass filter 129 is the needed tuning-voltage.
However, the circuit mentioned above is in an ideal situation, and in a real situation, unavoidably, there will be an offset current. FIG. 1C is a practical equal-effect model of FIG. 1B to illustrate the real situation, in which offset current 130 indicates the offset current produced by the transconductor 120 in the real situation, therefore a real transconductor can be equaled to an ideal transconductor 120 plus an offset current 130, and the current produced by this offset current 130 is marked Ioffset. Thereupon the current going through this differential to single converting transconductor 120 here becomes Ir+Ioffset. And making use of the said analyzing method, Equation 2 can be reached:                                                                         -                                  C                                      1                    ⁢                    p                                                                              C                                  2                  ⁢                  p                                                      *                                          (                                                      I                    r                                    +                                      I                    offset                                                  )                            Gm                                +                                                    N                *                                  I                  r                                                            C                                  2                  ⁢                  p                                                      *                          1              f                                      =                              0            ⁢                          
                        ∴                          ⇒                              Gm                                  C                                      1                    ⁢                    p                                                                                =                                    f              ⁡                              (                                  1                  +                                                            I                      offset                                                              I                      r                                                                      )                                      N                                              (Equation  2)            
It can be seen that the offset current produced by the Transconductor 120 will affect the value of the tuning frequency, and the error amount and the value are in direct ratio. And since the offset current changes because of the producing process of the Transconductor itself, the temperature of the environment, and the time factor, a frequency-tuning loop that is not affected by the offset current produced by the Transconductor is needed to upgrade the precision of the frequency-tuning loop.
Due to the several disadvantages in controlling the cut-off frequency of the traditional Transconductor-Capacitor filter in the background of invention described above, the invention provides a frequency-tuning loop used in the Transconductor-Capacitor filter in order to overcome the traditional problems.
The main purpose of the invention is to provide a frequency-tuning loop that is not affected by the offset current produced by the referential Transconductor in order to upgrade the precision of frequency of the Transconductor-Capacitor filter.
According to the purpose described above, the invention provides a frequency-tuning loop used in the Transconductor-Capacitor filter, comprising a transconductor; the positive inputting end of this transconductor connects to the switch 1a and 1b, with the other end of the switch 1a connecting to a positive current supply and the other end of the switch 1b connecting to a negative current supply; the negative inputting end of the transconductor links connects to the switch 1axe2x80x2 and 1bxe2x80x2, with the other end of the switch 1axe2x80x2 connecting to a negative current supply and the other end of the switch 1bxe2x80x2 connecting to a positive current supply. The action of switch 1a accords with switch 1axe2x80x2, the action of switch 1b accords with switch 1bxe2x80x2, and the action of switch 1a and 1axe2x80x2 and switch 1b and 1bxe2x80x2 occur alternately.
The outputting end links connects to the switch 2a and 2b respectively, with the other end of the switch 2a connecting to the first capacitance, switch 3a, switch 3bxe2x80x2, and the other end of the switch 2b connecting to the second capacitance, switch 3axe2x80x2, switch 3b. The on-and-off action of the switch 3a and 3axe2x80x2 is followed with that of switch 1a, the on-and-off action of the switch 3b and 3bxe2x80x2 accords with that of switch 1b. 
In the following is the integrated circuit consisting of the integrator, the third capacitance, and the fourth capacitance. The third capacitance is to store the charges transmitted from the first capacitance, and the fourth capacitance is to store the charges transmitted from the second capacitance. The positive inputting end of the integrator connects to the other end of the switch 3axe2x80x2 and the switch 3b, and the negative inputting end of the integrator links connects to the other end of the switch 3a and the switch 3b. A differential to single converter is connected to the outputting end of the integrator in order to transform the differential signal output by the integrator to single end signal and then a low-pass filter is connected to the outputting end of this differential to single converter.